SkepticblogSkepticblog logo banner

top navigation:

MPG vs L/100km

by Brian Dunning, May 24 2012

Photo:Trounce/Wikimedia Commons

One disadvantage of living in a country stuck using non-metric measuring systems is that we’re also stuck using a few related measures. One of the most familiar of these is mpg, miles per gallon. Many other countries use a newer standard, l/100km, liters per 100 kilometers. The salient difference between the two is not merely that one is metric and the other is not; it’s that they are multiplicatively inverted from one another. One gives fuel per distance, while the other gives distance per fuel.

When we talk about a car’s fuel economy, what we want to know is how much fuel does it use, not how far does it go. mpg answers the latter question, while l/100km is what gives us the answer we want. When we talk about a car that gets great fuel economy, we tend to speak instead in terms of how far it will go. This is the inverse of what we are trying to communicate.

The problem with using mpg — distance over volume of fuel — is that the relationship is not a flat line. It’s deceptive. Values at the lowest end of the mpg scale — where most of us scoff at all such cars — correlate to consumption numbers that are much further apart than those at the high end of the scale.

For example, imagine two cars that get 14 mpg and 17 mpg. Most of us look at them and say they’re both unacceptable. However, consider two cars that register 33 mpg and 50 mpg. Who among us would not clamor for the 50 mpg car, believing it to be far more fuel efficient than the 33? The fact is that in both pairs of examples, the car with the higher figure saves 1 full gallon of fuel on a 100-mile trip. Going from a 14 mpg car to a 17 saves exactly as much fuel (and carbon) as going from a 33 to a 50.

The advantage in using a linear scale of fuel consumption is that it tells us exactly what we want to know, without disguising the reality behind an invisible curve.

An increasing number of Monroney stickers — those mandatory window stickers on new cars that include the mileage among other things — are beginning to show l/100km values, though it’s still in a small font below the larger combined EPA mpg number. Still, it’s a step in the right direction.

65 Responses to “MPG vs L/100km”

  1. MikeB says:

    This isn’t such a big deal. The lines are nearly parallel. They measure the same thing in different ways.

    • Deen says:

      Uhm, one line is US gallon, and the other is British gallon…

      And yes, they do measure the same thing, it’s just that one is easier to use when judging the fuel consumption of a car.

      • MikeB says:

        Says who?

        Much ado about nothing.

      • Deen says:

        You didn’t really read the article, did you?

      • Other Paul says:

        I’m actually wondering what that graph is doing here at all. It’s not particularly helpful in demonstrating the problem under discussion and may even easily mislead the unwary into a category error.

      • Deen says:

        Especially when it’s presented without a caption or legend.

      • Tobias says:

        Dude, it’s for comparing d(y) over d(x), illustrating the point BD makes.

        Extrapolating the function, it becomes apparent that there is (practically) no consumption difference between, say 200 mpg and 300 mpg.

        And they say car salesmen are dishonest?!?!

      • tmac57 says:

        If you were on a deserted road 300 miles from the next fueling station,and you had only 1 gallon of gas,which would you rather be driving,a vehicle that gets 300 mpg,or 200 mpg? Hardly makes a difference right?

      • Tobias says:

        Are you a car salesman tmac57?

      • tmac57 says:

        Only a car salesman could convince someone that a 50% increase in gas mileage is insignificant.

      • says:

        Yeah, I get stranded hundreds of km in the desert all the time.

      • MadScientist says:

        I don’t see a problem at all. I think anyone who sees a problem needs some remedial arithmetic lessons. There is no deception whatsoever by either folks who use L/100Km or MPG. Plotting L/100Km against MPG doesn’t even make any sense – you’d expect a hyperbola as a result (and that is indeed the result) – but so what? Different folks looking for some measure of fuel efficiency have simply come up with 2 ways of obtaining the same results.

        If I wanted to travel, say, 80 miles and I knew my car got me 35 MPG, I know I need about 3 gallons to get somewhere. If I were 120Km from somewhere and I got 8L/100Km and I were lazy, I’d throw in 16L of fuel; if I felt like doing better calculations I’d figure a minimum 9.6L fuel for the trip (of course that would make the silly assumption that I would have average fuel consumption for the trip). If anyone is confused by using L/100Km or MPG as measures of fuel efficiency, they have a severe numerical handicap.

      • Johan™ Strandberg says:

        People that get stranded hundreds of miles into the desert with only one gallon of gas are a dying breed. Consider it evolution in action.

  2. logical says:

    Inverting a relationship doesn’t affect its linearity…

  3. Tom says:

    Mile > Kilometer

    What more do you need to know the American system is better?

    PS – It could be worse. In the UK, all the road signs are in miles, but the “petrol” comes in liters…errr…litres.

  4. Greg says:

    As an Aussie who uses l/100km I’ve often pondered if we’re using the wrong scale (inverted).

    The numbers are so low – 14+ for a 4wd, 12 for a big Aussie V6, 8 for a general 4-cylinder, 4-6 for little fuel-sippers that i think people don’t think of big cars as being much different to the small ones.

    Psychologically we don’t feel that twice as much fuel is as big a difference when 6 (2-door hatch) and 12 (family station-wagon) are compared.

    mpg may drive better behaviour in deciding if you really do need such a big car.

    (that was harder to write than necessary because we use very different languages to describe car classes – and have different models)

    • Morg. says:

      Every time you use non-scientific units, a kitten dies.

      So yes, I believe you were using the right units.

      As for the scale, if your brain cannot process that 12 = 6*2 , I don’t think fuel consumption is such an issue.

      All in all the simple solution is to come live in Europe for a while.

      Oil is so expensive a 2x difference will seem like a big deal, and as a plus we do not use any medieval measurement units.

  5. LawnBoy says:

    I completely agree with Brian – it would be much better to focus on the amount of Gas used.

    Here’s a (unfrequently-updated) site that promotes this idea:

  6. Max says:

    An infinite mpg car (e.g. solar powered) saves 1 gallon of fuel on a 100-mile trip compared to a 100 mpg car.

    • gabe says:

      0 L/100km === infinite MPG

      both equally express the idea that no fuel is being consumed. But the infinite MPG gives the false idea that the car moves by nothing. 0 L/100km just says the truth, that no fuel is used, nothing else.

    • MadScientist says:

      Absolutely – which is why I find Brian’s article irrelevant. However, judging by the other reply to your post, I see that some people really do need to review their arithmetic.

  7. Somite says:

    “The fact is that in both pairs of examples, the car with the higher figure saves 1 full gallon of fuel on a 100-mile trip. Going from a 14 mpg car to a 17 saves exactly as much fuel (and carbon) as going from a 33 to a 50″.

    This is the type of argument that may lead someone to argue “mileage is not that important. Go ahead and buy the 14 mpg car because you would only be saving a gallon”. Let’s not even discuss that any savings is significant. These are the amounts of fuel volume each car would consume in gallons:

    14 mpg = 7.14
    17 mpg = 5.88
    33 mpg = 3
    50 mpg = 2

    The correct statements when comparing these cars are proportional . For example; “the 50 mpg car uses about 4.5 times the gas as the 14 mpg car” or the “50 mpg car uses 2/3 of the fuel needed by the 33 mpg car”. If you really want to compare fuel usage between two vehicles go to and compare amount of petroleum needed to operate. That cuts through any variables like diesel or hybrid. Diesels are specially onerous since it is just concentrated gasoline.

    • Max says:

      To drivers, the difference in fuel consumption is more relevant than the ratio.
      An infinite mpg car is infinitely times more efficient than a 33 mpg car, but what matters to the driver is that it saves 3 gallons per 100 miles.
      Expressing efficiency in gallons per mile makes it easier to see the difference, and doesn’t make it any harder to find the ratio.

      • Somite says:

        But saying that it “just” saves 3 gallons may be misleading. Over a lifetime a car is driven a lot more than 100 miles. With large numbers small proportional changes result in very large absolute differences.

        For example even though a 50 mpg car only saves 1 gallon of gas per 100 miles when compared with a 33 mpg car it will save 1/3 of the total gas over the lifetime of a car. This is a very large number. Conisder the US consumes 140 billion gallons of gasoline a year. Reducing consumption by 1/3 of 140 b is 46 billion gallons of gas saved.

    • Alex says:

      “This is the type of argument that may lead someone to argue “mileage is not that important. Go ahead and buy the 14 mpg car because you would only be saving a gallon”.”

      Would it? All empirical evidence shows the opposite: countries that use L/100km tend to have much more efficient cars on the road.

      • Donald Prothero says:

        All countries which use L/100 km have high gas prices, too, which is more germane to why people care about mileage.

  8. Nicholas says:

    I disagree with Brian. The question we want to know is neither ‘how much fuel does it use’ nor ‘how many miles it will go’ but rather ‘how much time can I wait between fillups’. If we assume that we drive at a consistent rate over time, then we can equate time with miles, and then miles-per-gallon substitutes for what we really want to know: days-per-fillup. If I double my MPG, I can double the time between fillups! Of course, meters-per-liter would be equally valid in that sense.

    My true feeling, to be honest, is that Brian has gone through the trouble of making a point which is literally not worth his breath. MPG and GPM are exactly the same to anyone who understands fractions. I suspect that Brian’s penchant for GPM is really a transposed penchant for the metric system. [And that is fair, but a different question.]

    Also, a shoutout to the pedants: the fuel savings on a 100 mile trip is one-quarter larger for 14 -> 17 than for 33 -> 50.

    Brian, thank you for Skeptoid and all your other efforts. I am a contributor, and will be again; your hard work makes the world a better place.

    • Max says:

      A bigger gas tank can increase the range between fill-ups without any change in efficiency.

      • MadScientist says:

        That’s not true – efficiency will be lower because you have to carry the extra fuel and the extra weight of the larger tank. I don’t have any idea how much lower, but the efficiency will be lower.

    • BKsea says:

      Nicholas: “MPG and GPM are exactly the same to anyone who understands fractions.”

      Which is about 1/10 (i.e. 20%) of the American population.

      I think this really comes down to a perception question. If people focus on absolute differences, they may make poor choices regarding low MPG cars. That is 17 and 14 are only 3 apart so no big difference. If using l/100KM, the difference might be more apparent.

      On the other hand, if people focus on relative difference (i.e. 17 is 21% higher than 14), that may seem like a bigger difference.

      What we want is a measurement that generates the desired intuitive response. It is not clear which one it is.

  9. Max says:

    Should we also talk about hours per mile instead of mph, because after all, we care about how long it takes to travel a given distance, not how far we can go in a given amount of time.

    • Max says:

      Accelerating from 13 mph to 15 mph saves as much time as accelerating from 33 mph to 50 mph.
      Runners talk about the 5-minute mile, right?

  10. Hugh Brown says:

    You left out the most interesting part: how to convert between the two.

    (miles/US gallon) * (liters/100 km) is a constant — 235, more or less.

    Similarly, (miles/imperial gallon) * (liters/100 km) is a constant — 282, more or less.

    Take the measure you have and divide into the relevant constant. So 20 miles per US gallon is about 11.75 liters / 100 km (because 20 * 11.75 is 235).

    • tmac57 says:

      I’ve got an app for that ;)

    • Morg. says:

      You know you could multiply that by the mass of the earth in Kg times the mass of the moon in g and divide it by the mass of the sun in Kg times Pi and obtain exactly n times your number, where n is a number defined with exactly three wikipedia searches.

      Everything can be made complex, but the simple way is the best one.

      Drop the mpg measurement altogether. no conversion, no medieval units, no crap.

  11. Tom says:

    What the fuck is a “gallon”? Some sort of ancient pirate shit?

  12. JeffyPooh says:

    My car gets significantly better fuel economy on longer trips than shorter trips. My daily commute to work is extremely short, just a few minutes and only one exit up the highway. I could improve my car’s average fuel economy figures by simply moving to a different house much further away from work.

  13. Bobby Newmark says:

    I think you’re just used to one way of doing things and are trying to justify it. An anthropological study showed that a tribe of hunter gatherers without a formal number system naturally think of numbers as ratios, not as differences. For example, when asked what is halfway between one and nine, they answered three (1 * 3 = 3, 3 * 3 = 9). It is not unnatural or difficult for people to think this way, and your claim that it is more important for a person to know the absolute difference is unfounded.

    • Max says:

      It’s natural, but it’s not always rational. Like, people would drive a couple extra miles to pick up a $20 item for free, but they wouldn’t bother driving that distance to get a $1020 item for $1000. Unless they already drove 20 miles, in which case a couple more miles sounds relatively little. Why is driving a couple miles worth $20 in one case but not the other?

    • Max says:

      The tribe takes square roots? Really?

      • Max says:

        And if they do take square roots, why did the anthropologists translate it into English as “halfway” instead of as “square root”?

  14. Morg. says:

    “newer standard” ?

  15. wojtek says:

    As a person who comes from a European country, using L/100km as a general measurement, and now I’ve been living for a few years in a country(UK) that uses MPG, I noticed one thing – British people have absolutely no idea how much fuel their cars actually use. They all know the MPG of their cars, but that kind of measurement is absolutely useless for actual life, it’s only a way of saying which car has a better fuel consumption than a different one. If I buy 100 litres of fuel at a petrol station, I know exactly how far I can get with that if my car uses 15 litres/100km. If I go to town and back, knowing that the trip there and back is 20km, I know precisely how much it cost me to get there and come back home. British people on the other hand, sit in their 40MPG cars, they go somewhere and they have no idea how much petrol they used. Sure, after 40 miles,they used….a gallon of petrol. But nobody here knows how much a gallon is, entire Europe buys petrol in litres. And for all Americans – after conversion to US dollars, our petrol last time I checked was $9.50 per gallon. And I hardly see anybody complaining. So please get over it.

  16. JeffyPooh says:

    Most Hunter-Gatherer tribes (those “without a formal number system”) would reply, “What’s this ‘nine’ of which you speak?”

  17. George Crews says:

    Tongue firmly in cheek, let me see if I can list the points you’re trying to make:

    1) I’m having a hard time understanding mpg, and I find l/100km easier to understand.

    2) Other people are too stupid to really understand mpg, and the “newer” standard of l/100km will make it easier for those morons.

    3) I hate non-decimal measurement systems (except for time, of course) and so I’m inventing any disadvantage in such non-decimal systems that I possibly can to justify my dislike.

    4) Mpg was invented not by engineers who knew what they were doing–but by car salesmen who want to sell you a gas guzzler.

    None of those actually your point? Then a modest proposal: if you are having issues understanding fuel economy for your next car, plug the following formula into your smartphone’s calculator:

    gallons per 100 miles = 100 / mpg

    Now you can compare just as effectively as l/100km.

  18. CountryGirl says:

    Many years ago I had a motorcycle that got about 66 mpg. I rode it 100,000 miles. Criss-crossed the country numerous times. But experience and reality made me get rid of it. Too dangerous. The one thing “most” 50 mpg cars have in common is they are too dangerous. Thankfully they are building cars that can reach about 30 mpg that are still large enough and heavy enough to be safe in a crash (yes I am aware there are other factors involved in safety). I will stick with the larger safer cars over the minis.

    • John Greg says:

      “The one thing ‘most’ 50 mpg cars have in common is they are too dangerous.”

      Huh? And why is that?

      • wojtek says:

        Because usually they are tiny and in case of an accident you are getting squeezed inside like a fresh orange.

      • Somite says:

        Sounds like we should be aiming towards less irrationally large cars on the road. It should worry you that you would drive a vehicle that could hurt someone else!

      • Max says:

        Small cars tend to be less safe in collisions with similar-sized cars and stationary objects because they have less material in front to soften the impact, but their safety rating should reflect this. However, even small cars with a high safety rating are not safe in a collision with bigger car. This would be less of a problem if everyone drove similar-sized cars.

      • tmac57 says:

        Well,at least they make a smaller target, right?

      • wojtek says:

        You might convince people to stop driving SUVs,but there will always be lorries on the road. And in case you hit a wall or a road-side barrier you also have a much smaller lower chance of surviving in a tiny car.

      • Somite says:

        This is the type of low incidence but high morbidity event that I don’t think is worth planning for. Chances are low that you in particular will be in a serious car crash. Is it worth emitting all those gases and paying for all that gas to prevent that unlikely of an event?

        It is just like walking around with a gun in case there is a shooting.

      • Jimmy Russells says:

        “It is just like walking around with a gun in case there is a shooting.”

        So you’re saying that cops shouldn’t carry guns? What if there is a shooting? How would you stop the shooter?

      • Somite says:

        Cops have the training and the communication systems to make carrying guns worthwhile.

        That is the point. It is so extremely unlikely to be involved in a shooting that carrying a gun and all its adverse effects are not with it.

      • Jimmy Russell says:

        “…carrying a gun and all its adverse effects are not with it.”

        What adverse effects are there to carrying a gun? Also, not all of us are content to hope and pray that the police arrive before a shooter runs out of ammo and/or victims.

  19. Peter Maas says:

    Great post, by the way I am an engineer & I want to join a community where I can get spiritual as well as I can help others. I heard about global community communications alliance in AZ. As far as I got know it is the nonprofit organization.

    • BBunsen says:

      Peter, please stick to the topic, and please don’t spam this site with your New Age claptrap.


  20. WAL says:

    I am a big fan of this site. This however is the dumbest thing I have seen in a while. I am surrounded by foolishness in most of what I see from the media I don’t want to see it here.

    There is no issue worthy of discussion. Maybe if we stated the MPG in Latin it would be better. If one wants to look at this from a different perspective why not consider how many people are being moved. Would a bus be criticized for it low liters per kilometer?

    • markx says:

      I second this comment.

      • Tobias says:

        I also concur with markx’ assentment that WAL is a big fan of this site.

      • markx says:

        Right .

        Yes, I regretted my inane comment as soon as it was made.

        But, though the topic could also be labelled as such, I guess any debate is good debate.

        Perhaps I felt I just needed to get a comment in quickly before I was blocked … again.